The present invention relates to printers, in particular printers of the type including a moving image-carrying belt, and a plurality of toner image producing stations for sequentially depositing a plurality of toner images in register with each other on the surface of the image-carrying belt to form a composite toner image thereon.
U.S. Pat. No. 5,805,967 (De Cock et al./Xeikon NV) describes a single pass, multi-color electrostatographic printer. A plurality of toner images of different colors are electrostatically deposited sequentially in powder form in register with each other from a plurality of printing stations onto an image-carrying member which is in the form of a intermediate transfer belt. The so-formed composite toner image is then transferred to a silicone coated metal transfer belt which is brought into contact with a substrate. The composite toner image is thereby transferred to the substrate.
The intermediate transfer belt may have a toner image-carrying surface formed of an electrically non-conductive material selected from polyethylene terephthalate, silicone elastomer, polyimide (such as KAPTONxe2x80x94Trade Mark), and mixtures thereof.
It has been found that where several toner images are transferred electrostatically to the intermediate transfer belt, some distortion of the composite image may occur upon transfer to the transfer belt, revealing itself in badly transferred areas in the final printed product.
It is an object of the present invention to provide a printer of the type described in which these disadvantages can be overcome.
We have discovered that this objective, and other useful advantages, can be obtained when the image-carrying belt is formed of a non-conductive material having specified electrical properties.
According to the invention there is provided a printer including a moving image-carrying belt, and a plurality of toner image producing stations for sequentially electrostatically depositing a plurality of toner images in register with each other on the surface of the image-carrying belt to form a composite toner image thereon, wherein the image-carrying belt is formed of an electrically non-conductive material having a mean volume resistivity of from 108 to 1011 ohm m.
In a preferred embodiment of the invention, the printer also includes a transfer member for transferring the composite image from the image-carrying belt to a substrate.
While not wishing to be bound by theory, we believe that in the case of a multi-color printer in which a plurality of toner images of different colors are deposited by electrostatics on the intermediate transfer belt in turn, the voltage which needs to be used to deposit the toner at a given printing station depends upon the voltage which is retained upon the belt surface and the toner deposited thereon at the preceding station or stations. Thus, as the belt progresses from one printing station to the next, the overall charge on the belt would tend to build up. If, as result of such a charge build-up, a discharge should take place between the belt and an imaging drum at one of these printing stations, or possibly between the belt and the transfer member, such a discharge would be liable to disturb the images on the belt. Thus, where several toner images are electrostatically deposited on the image carrying belt, charge on the belt can build up to the point where sparking or other uncontrolled discharge takes place, due to Paschen discharge where the image-carrying belt and the latent image carrier are being separated or are coming into contact with each other. By selecting the electrical properties of the image-carrying belt this excess charge is able to dissipate in a controlled manner between printing stations, the electrostatic voltages required for toner transfer at subsequent printing stations are thereby reduced and uncontrolled discharges are thereby avoided.
We are aware of U.S. Pat. No. 5,823,017 (Soga et al./Ricoh) which recommends the use of an intermediate transfer belt formed of a material having a volume resistivity of 108 xcexa9cm to 1010 xcexa9cm (106 to 108 xcexa9m), and a surface resistance of 107 to 1013 xcexa9, in a printer of a configuration different to that of the present invention. According to column 11 lines 37 et seq. of this patent, a device for discharging the intermediate transfer belt is not necessary because such a belt can be easily discharged by a ground roller. It is stated that volume resistivities lower than 108 xcexa9cm (106 xcexa9m) would prevent the toner image from being transferred to the belt while volume resistivities greater that 1010 xcexa9cm (108 xcexa9m) would result in the need for an extra discharging device in addition to the ground roller.
We have found that where the resistivity is too low, such as a volume resistivity of about 104 ohm m or less, the image-carrying belt cannot sustain a charge sufficiently for efficient transfer of the toner image thereto. When the resistivity of the image carrying belt is too high, such as a volume resistivity of about 1013 ohm m or more, excess charges are not able to dissipate in a controlled manner and some discharge may occur.
The image-carrying belt is ideally formed of a single layer of the electrically non-conductive material, the mean volume resistivity being from 108 to 1011 ohm m volume throughout the thickness of the belt.
In the present context, volume resistivity is measured according to ASTM D257. Preferably, the non-uniformity of the resistivity of the image-carrying belt is not more than xc2x11 order of magnitude.
It is desirable that the material of which the image-carrying belt is formed is ozone resistant. In the present context, ozone resistance is measured by the standard NEN-ISO 1431/1-1990, in which the sample is subjected to a tensile stress of 5 MPa in an atmosphere containing 5 ppm ozone at 55% relative humidity.
It is also desirable that the material of which the image-carrying belt is formed has a low moisture adsorption. If moisture is adsorbed onto the surface of the image-carrying belt, the electrical properties thereof can change and in particular the mean volume resistivity can fall. We prefer that the material of which the image-carrying belt is formed has mean volume resistivity of at least 108 ohm m when measured in an atmosphere of 45% relative humidity. By using such a material, the need for extreme environmental conditioning measures to be taken in the printer is much reduced.
The surface of the image-carrying belt preferably has a hardness of at least 63 Shore D or equivalent. If the hardness is insufficient, the surface will be more susceptible to damage, e.g. by scratching.
The surface of the image-carrying belt, in contact with the image, preferably has a surface energy as low as possible to reduce the formation of toner filming and to reduce contamination from (sticky) components released by the printed substrates onto the surface of the transfer member. Most preferably, the image-carrying belt has a surface energy of less than 60 dynes/cm. Furthermore, a lower surface energy for the image-carrying belt is advantageous in maximizing the transfer efficiency and avoiding print defects in the transfer of the composite image to the transfer member.
Another important requirement with respect to the surface conditions is the roughness. The Ra-value for the surface of the image-carrying belt is preferably less than 0.5 xcexcm to minimize the image noise and to maximize transfer efficiency.
The electrically non-conductive material from which the image-carrying belt is formed may be selected from within the classes of polyesters, polyethers, polyamides, polyimides and copolymers thereof. The material used can be homogeneous. Alternately, during the formation of the material, components can be added to modify the resistivity and/or the thermal properties and/or other properties of the material.
The image-carrying belt is preferably in the form of a non-woven continuous belt, especially a seamless belt, or a belt with a so-called invisible seam. Seamless belts may be formed by a centrifugal casting technique, as known in the art.
It is desirable that the material of which the image-carrying belt is formed is heat stable, that is the material can be melted and re-solidified without significant lasting chemical change occurring. With such heat stable materials the production of a seamless belt, or a belt having an xe2x80x9cinvisiblexe2x80x9d weld, is greatly facilitated. A seamless belt, or a belt with an invisible weld, is essential when the substrate is in the form of a web and the printer is intended to print images of indeterminate length.
The belt suitably has an average thickness of between 100 and 150 xcexcm. If the thickness of the belt is too low, the stiffness is reduced making the belt vulnerable to deformations. If the thickness of the belt is too high, the stiffness and low transfer fields induce transfer defects such as voids and a loss of efficiency.
Ideally, the belt has a tensile modulus of greater than 0.5 Gpa, most preferably greater than 2 Gpa, as measured by ASTM D 790 IB. If the tensile modulus is too-low, it will result in registration problems between the individual toner images because relatively small tensile stress variations in the image-carrying belt can result in significant uncontrolled belt elongations between the toner image producing stations.
Also, the belt should have a tear resistance of at least 200 N/mm. If the tear resistance is too low the lifetime of the belt is decreased.
The following table illustrates some materials suitable for use as the image-carrying belt, together with some materials of similar composition of which some properties are not or improperly controlled, since they fail to meet one or more of the target characteristics. Characteristics are measured at a material thickness of 110 to 150 xcexcm, at ambient temperature.
Means for tensioning the image-carrying belt may be provided in order to ensure good registration of the toner images thereon and to improve the quality of transfer of the composite toner image therefrom to the transfer member. Means for controlling the transverse position and movement of the image-carrying belt may also be included.
Each toner image producing station may include a rotatable endless surface, such as a drum having a photosensitive surface, on which an electrostatic latent image is formed, a developing unit for developing the electrostatic image to form a toner image on the drum surface and means for transferring the toner image to the image-carrying belt. Each image producing station will generally include an electrostatic device for depositing the toner image on the image-carrying belt. The electrostatic device can be in the form of a transfer corona or a charged transfer roller. The image-carrying belt is preferably in contact with the drum over a wrapping angle of more than 5xc2x0 so that adherent contact between the image-carrying belt and the drum surface enables drive to be reliably transmitted from the image-carrying belt to the drum. The reliability of this transfer is enhanced by tensioning the image-carrying belt.
The transfer member is provided for transferring the composite image from the image-carrying belt to a substrate. To achieve this, the composite image on the transfer member is heated to tackify the image before contact with the substrate. This heating may be achieved by heating the image directly, for example by use of a radiant heater positioned adjacent the surface of the transfer member, or indirectly, for example by heating the transfer member itself. Where the temperature of the composite toner image and the pressure exerted between the transfer member and the substrate at their region of contact are sufficient, no further fixing of the image on the substrate is required.
The transfer member may include an outer surface formed of a material having a low surface energy, for example silicone elastomer (surface energy typically 20 dyne/cm), polytetrafluoroethylene, polyfluoralkylene and other fluorinated polymers. The transfer member is preferably in a form having a low mass. For this reason, while the transfer member can be in the form of a transfer roller or drum, it is preferably in the form of a transfuse belt, for example an endless metal belt of 80 xcexcm thickness coated with 40 xcexcm thickness silicone rubber.
The transfer of the composite toner image from the image-carrying belt to the transfer member is more difficult to achieve if the transfer member has a relatively low surface energy. Therefor, it would be preferable to transfer the composite toner image from the image-carrying belt to the transfer member by electrostatic means.
Drive to the image-carrying belt is preferably derived from the drive means for the transfer member, by making use of adherent contact between the image-carrying belt and the transfer member causing the image-carrying belt and the transfer member to move in synchronism with each other. Adherent contact between the image-carrying belt and the image producing station drums may be used to ensure that the drums move in synchronism with the image-carrying belt. The image-carrying belt preferably passes over a guide roller positioned in opposition to the transfer member to form a first nip or contact region therebetween.
Means for cleaning the image-carrying belt are preferably provided after contact with the transfer member.
The transfer member may be positioned in opposition to a counter roller to form a second nip therebetween, through which the substrate path passes. After the transfer of the composite toner image therefrom, the transfer member is cooled, preferably forcibly, to lower the temperature of the transfer member to the optimum temperature for the transfer of further composite toner images thereto. Without such cooling, there is a risk that contamination originating from the substrate might transfer back to the image-carrying belt to disturb the performance of the latter. Furthermore, transfer defects can occur at the transfer between the image-carrying belt and the transfer member if the temperature is too high relative to the softening point of the toners. Although the image-carrying belt may be cleaned after its contact with the transfer member, higher temperatures are more likely to result in filming at the cleaning station.
Alternatively, the image-carrying belt may be separately driven which allows for a much smaller contact zone between the image-carrying belt and the heated transfer belt. As a consequence, there is less heat exchange between the two belts which allows at least for a less rigorous cooling of the transfer belt. Cooling of the transfer belt may even be omitted, particularly when using a heat stable image-carrying belt according to the present invention which assures that the volume resistivity of the belt remains in the range from 108 to 1011 ohm m even when being exposed to heat in said contact zone. Furthermore, by using a belt with an average thickness in the range from 100 to 150 xcexcm, belt deformation can be avoided in the contact zone with the heated transfer belt.
Where the temperature of the composite toner image and the pressure exerted between the transfer member and the substrate at the second nip are insufficient, means for further fixing and glossing of the image on the substrate may be provided downstream of the second nip.
The substrate may be in the form of a web, but the invention is equally applicable to substrates in sheet form, the device then being provided with suitable sheet feeding means. When the substrate is in the form of a web, web cutting means, optionally together with a sheet stacking device may be provided downstream of the second nip. Alternatively, the web is not cut into sheets, but wound onto a take-up roller. The web of substrate may be fed through the printer from a roll.
The printer according to the invention may also be part of an electrostatic copier, working on similar principles to those described above in connection with electrostatic printers. In copiers however, it is common to expose the rotatable endless surface exclusively by optical means, directly from the original image to be copied.